wherein a thickness of a TM slab measured by a Rietveld method in a crystal structure of a space group R-3m by an X-ray diffraction (XRD) analysis is in a range from 2.13 Å to 2.15 Å,

wherein a crystal size in a (003) plane defined by Equation 2 below is in a range from 150 nm to 300 nm:

wherein, in Equation 2, L₀₀₃ is the crystal size (nm) in the (003) plane, K is a shape factor, λ is an X-ray wavelength (nm), B₀₀₃ is a full width at half maximum (rad) of a (003) plane peak from the XRD analysis, and θ represents a diffraction angle (rad).